In a study, both black and white patrons at a moderately priced Midwestern restaurant tipped black servers less than their white counterparts. This disparity was found in spite of the fact that patrons reported being more pleased with the black servers’ work.

Considering the fact that tips make up more than half of waiters’ income (at least according to a 2012 survey), this is more than a symbolic slight. It suggests black servers, on average, take home significantly less money than their white colleagues.

The study, by sociologists Zachary Brewster of Wayne State University and Michael Lynn of Cornell University, confirms the results of similar research published in 2008. The earlier study, however, was conducted at a restaurant in the U.S. South.

"Our results indicate that both white and black restaurant customers discriminate against black servers by tipping them less than their white co-workers."

The establishment at the center of this new study is in the Midwest. The similar results, the researchers write in the journal Sociological Inquiry, suggest “this phenomenon is not unique to specific locales.”

Brewster and Lynn surveyed 394 people who had just eaten dinner. Patrons were asked the amount of their bill, the size of the tip they left, and the race of their server.

In addition, the diners rated their server’s “appearance, friendliness, attentiveness, and promptness,” and noted such specifics as whether he or she “made them feel comfortable and welcome” and “seemed to sincerely care about their dining experience.”

The disheartening findings: “Our results indicate that both white and black restaurant customers discriminate against black servers by tipping them less than their white co-workers.”

Furthermore, “to the degree that there are interracial differences in serving skills, black servers in this study are perceived to provide better service relative to that provided by their white co-workers,” the researchers report. “Black servers were rated more favorably than white servers across each of three unique indices measuring service skills.”

Thus, after their quality of work was taken into account, “the disparity between tips given to black and white servers was enhanced rather than attenuated.”

Demographics suggest there weren’t a lot of overt racists among the restaurant’s patrons. (The average patron surveyed was a 43-year-old, college-educated female; 63 percent were white.)

Rather, the researchers point to unconscious bias as a “sound, theoretically informed explanation” of their findings.

“Tipping decisions are not only made quickly, at the end of the dining encounter, but are also to some degree made without much thought,” they write. “Consumers tend to round up or down from the calculated tip percentage they leave their servers, and such adjustments seem to be made without much conscious deliberation. Thus, it makes theoretical sense that tipping decisions might be unconsciously influenced by implicit racial biases.”

Why this behavior extends to black customers is less clear. But the researchers point to anecdotal reports that blacks sometimes over-tip white servers for fear of perpetuating the stereotype of “blacks don’t tip well.” If so, that would contribute to the imbalance the researchers found.

“Like all social inequalities,” the researchers conclude, “the underlying causes of such disparities are likely to be multifaceted and complex.” But as this research confirms, they are also real, and they do real, if unintended, economic harm.

So here's a tip: The next time you're throwing down 15 or 20 percent before heading out of a restaurant, take a moment and think about how much you are leaving, and why.